Enemies with Health
Before you start, you will need either a Maze or Platform engine with a character
that can attack.
You will also, and obviously, need at least one type of enemy.
Download the the Basic Platform Shooter engine here
Giving your Enemy Health
We will keep track on an enimies health using a variable, with varibales, before we can use it, we must create it when the object is created.
Open your Enemy object and Add or Open the Create Event 
From the Control Tab, add the, Set Variable Action 
This new variable will keep track of how much health the enemy has.
Set the variable name as, hp
Set the value as, 5, or how ever much health you want to give it.
You can't name the variable, health, because that variable is already used by game maker to keep track of the player's health.
Now we have made a variable to store the enimies health, we have also set it, now, we need to reduce it when the enemy is hit with an attack, in this example, the bullet.
Reducing the Health when Hit with an Attack
Create a collision event with the bullet or weapon that will hurt your enemy. 
In this example, i'm using a bullet.
From the Control Tab, add the, Set Variable Action
Set the variable name as, hp, because we want to change the hp
Set the value as, -1 and tick relative, this will take 1 away from the hp value
If you are using a bullet in the collision,
From the Main1 Tab, add the, Destroy
Instance Action and Select Other 
This will delete the other object invovled in collision, which is the bullet.
Checking the Health and Destorying the Enemy if No Health
At the moment, your enemy loses health when it is attacked. We want to destory the enemy when it has no health left. We can't use the No more Health Event because this is for when your main character has no more health, instead, we have to check the hp variable we created on the Enemy.
Still in the Collision event with the bullet or weapon,
From the Control Tab, Add the, Check IF Variable has a Value Action 
We want to check if the hp is less than 1, meaning there is none left.
Set the variable as, hp, because we want to check how much hp is left.
Set
the value as 1 and the operation as smaller than.
This will either be true or false, if it is true, we want to delete the enemy object.
After the IF Variable has a Value Action, from the main1 tab, add the Destory
Instance Self Action
The collision event with the bullet should now look like this,
This will take 1 away from the hp and then destroy the bullet that hit the enemy. Then, it will check if the hp is less than 1, if it is, it will destory the enemy.
Download working example